Matt - to answer one of your queries there is a handy grid converter here:
http://www.streetmap.co.uk/gridconvert.html
I have a GPS handset which has helped me find some obscure sites but in a recent British Archaeology there was a website with a free downloadable waymark plotter that was tested to be quite accurate. Useful for making your own plots of circles or working out the relationship between sites in a particular area. I need to make an eBay purchase of the GPS download cable then I'm well away.
Grendel